Continuation of work to demonstrate, separate and identify the stromal factors of the hemopoietic microenvironment and to determine their cellular and chemical identities as well as their mechanism of action. In addition to further work elucidating these factors in the normal state and in a genetically determined mouse anemia, the proposed effect will examine their role in certain induced aplasias and leukemias, using animal models. Histologic studies by light and electron microscopes and radioactive iron uptake will be used to provide measurement data.